#269 chore: migrate from gopkg.in/yaml.v3 to go.yaml.in/yaml/v3

Migrates imports and go.mod module dependencies from legacy gopkg.in/yaml.v3 to maintained go.yaml.in/yaml/v3 (v3.0.5).

Resolves #201.

Summary of changes:

• Promoted direct module dependency on go.yaml.in/yaml/v3 v3.0.5 across go.mod manifests.
• Migrated package imports across the codebase from gopkg.in/yaml.v3 to go.yaml.in/yaml/v3.
• Bridged external query evaluators (jsonpath, yamlpath) in overlay with zero-copy node casting to maintain compatibility without performance degradation.
• All unit tests and go vet checks pass cleanly.

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Summary by cubic

Migrates the YAML library from the unmaintained gopkg.in/yaml.v3 to the maintained go.yaml.in/yaml/v3 fork, resolving #201.

• Updates all YAML imports and go.mod dependencies across the codebase, including cmd/openapi, openapi/linter/customrules, and jsonschema/oas3/tests, and aligns their root-module pseudo-versions.
• Bumps github.com/speakeasy-api/jsonpath to v0.7.0 and switches the jsonpath and yamlpath evaluators to go.yaml.in/yaml/v3, dropping the bridge code that converted nodes between the two modules.
• Encodes typed-nil legacy yaml nodes as null in sequencedmap.